Hello there, ring buddy.
If you don't need it to be new you could try finding vintage platinum bands in an art deco, georgian, or edwardian style. You can often find great deals at pawn shops or local jewelers if you are comfortable describing what you want.
Amazon Collection has some plain platinum bands in budget
This designer is over budget at $570 , but it is very interesting and in platinum: WARNING from Etsy
You can also find some great deals on eBay